Wavelength Control
Lasers are used for many applications. For example, lasers, such as KrF and ArF excimer lasers, are used in stepper and scanner equipment for selectively exposing photoresist in a semiconductor wafer fabrication process. In such fabrication processes, the optics in the steppers and scanners are designed for a particular wavelength of the laser. The laser wavelength may drift over time and, thus, a feedback network is typically employed to detect the wavelength of the laser and correct the wavelength as necessary.
In one type of feedback network used to detect and adjust the wavelength of a laser, an etalon receives a portion of the emitted light from the laser. The etalon creates an interference pattern having concentric bands of dark and light levels due to destructive and constructive interference by the laser light. The concentric bands surround a center bright portion. The position of the bright center portion of the interference pattern is used to determine wavelength to a relatively coarse degree, such as to within 5 picometers (pm). The diameter of a light band is used to determine the wavelength of the laser to a fine degree, such as to within 0.01-0.03 pm. The width of a light band is used to determine the spectral width of the laser output. The interference pattern is usually referred to as a fringe pattern. The fringe pattern may be optically detected by a sensitive photodetector array.
Various methods are well known for wavelength tuning of lasers. Typically the tuning takes place in a device referred to as a line narrowing package or line narrowing module. A typical technique used for line narrowing and tuning of excimer lasers is to provide a window at the back of the discharge cavity through which a portion of the laser beam passes into the line narrowing package. There, the portion of the beam is expanded in a beam expander and directed to a grating which reflects a narrow selected portion of the laser""s natural broader spectrum back into the discharge chamber where it is amplified. The laser is typically tuned by changing the angle at which the beam illuminates the grating. This may be done by adjusting the position of the grating or providing a mirror adjustment in the beam path. The adjustment of the grating position or the mirror position may be made by a mechanism which we will refer to as a laser wavelength adjustment mechanism.
In the prior art, the typical feedback network is configured to maintain the nominal wavelength within a desired range of wavelengths. Typical specifications may establish this range at values such as xc2x10.05 pm of a target wavelength such as, for example, 248,327.1 pm, as applied to the average of the wavelengths of a series of pulses referred to as xe2x80x9cpulse windowxe2x80x9d. A typical pulse window would be 30 pulses. Another typical specification is the standard deviation of the measured wavelength values for a series of pulses (such as 30 pulses). This value is referred to as sigma, "sgr", and is calculated using the standard formula for standard deviations. Also, sometime specifications are in terms of 3"sgr" which is merely three times the measured standard deviation. Typical 3"sgr" specifications may be 0.15 pm.
The limitations of acceptable optical lens materials for use with deep ultraviolet light at 248 nm and 193 nm wavelengths have meant that projection lenses for KrF and ArF lithography have been fabricated primarily with fused silica. Although fused silica is a very good lens material (high transparency, low thermal expansion, relatively easy to polish), the unavailability of a second material type with a different refractive index in projection lenses results in chromatic aberrations. Chromatic aberrations emerge since the index of refraction of any optical material changes with wavelength, and hence, the imaging behavior of a lens also varies with wavelength.
The detrimental effects of chromatic aberrations for an uncorrected lens can be mitigated only by using a light source with a very narrow range of wavelengths. Spectral line-narrowed excimer lasers have served this purpose for deep-UV lithography. Today""s lasers have bandwidths in the subpicometer range, providing nearly monochromatic illumination for refractive projection lenses. Nevertheless, although excimer laser bandwidths are small, the lack of chromatic correction in lenses means that the bandwidth cannot be ignored.
The bandwidth of the laser beam is typically made small by the use of line narrowing package referred to above. In the past, laser specifications have required the band width to be smaller than a specified value such as 0.5 pm. Specifications are also directed at the 95 percent integral bandwidth. A typical 95% I specification would be less than 1.2 ppm. However, recently integrated circuit manufacturers have noticed that the quality of their integrated circuits can be adversely affected by bandwidths which are substantially narrower than the bandwidths for which their optical systems were designed.
What is needed are techniques to control laser bandwidths within specified ranges rather than merely less than a specified width.
The present invention provides a technique for bandwidth control of an electric discharge laser. Line narrowing equipment is provided having at least one piezoelectric drive and a fast bandwidth detection means and a bandwidth control having a time response of less than about 2.0 millisecond. In a preferred embodiment wavelength tuning mirror is dithered at dither rates of more than 500 dithers per second within a very narrow range of pivot angles to cause a dither in nominal wavelength values to produce a desired effective bandwidth of series of laser pulses.